1. Field of the Invention
The present invention relates to a cavity transfer mixing extruder for uniformly mixing and extruding highly viscous materials, such as molten plastics or fluid rubbers, and, more specifically, to a cavity transfer mixing extruder having a stator provided with cavities of improved shape in the inner circumference and a rotor provided with cavities of improved shape in the outer circumference, and capable of achieving satisfactory extrusion of the materials.
2. Description of the Prior Art
Cavity transfer mixing extruders for uniformly mixing and extruding highly viscous materials, such as molten plastics or fluid rubbers, are disclosed in Japanese Patent Provisional Publication Nos. 60-107306 and 57-87344.
The cavity transfer mixing extruder disclosed in Japanese Patent Provisional Publication No. 60-107306 is illustrated in FIGS. 10 to 12. As illustrated in FIG. 10, the cavity transfer mixing extruder has a cylindrical housing 1, a cylindrical stator 2 fixedly fitted in the cylindrical housing 1, a cylindrical rotor 3 coaxially and rotatably extended within the stator 2. The rotor 3 is connected at one end thereof to an extruding member such as a screw shaft, and is driven in rotation by a motor.
As illustrated in FIG. 12, a cavity group 8 comprising cavity rows 6 arranged along the axial direction Y--Y and each having a plurality of cavities 4 arranged side by side along the circumferential direction X--X, and a cavity group 9 comprising cavity rows 7 arranged along the axial direction Y--Y and each having a plurality of cavities 5 arranged side by side along the circumferential direction X--X are formed in the inner circumference of the stator 2 excluding the opposite ends thereof and in the outer circumference of the rotor 3 excluding the opposite ends thereof, respectively. The respective cavities 4 and 5 of the cavity groups 8 and 9 are arranged with very small intervals so that the cavities 4 and 5 are distributed densely.
The respective cavities 4 and 5 of the succeeding cavity rows 6 and 7 of the cavity groups 8 and 9 are shifted by a distance lx along the circumferential direction in the respective directions of relative rotation of the cavity groups 8 and 9, namely, in a direction opposite the direction R of rotation of the rotor 3 for the cavity group 8 of the stator 2 and in the direction R of rotation of the rotor 3 for the cavity group 9 of the rotor 3, relative to the respective adjacent cavities 4 and 5 of the adjacent preceding cavity rows 6 and 7, respectively.
The cavities 4 of the cavity rows 6 of the cavity group 8 of the stator 2 are shifted by a distance ly corresponding to half the axial pitch (i.e. interval along the axial direction Y--Y) of the cavity rows 6 and 7 relative to the corresponding cavities 5 of the cavity group 9 of the rotor 3. Accordingly, the cavities 4 of each cavity row 6 of the cavity group 8 overlap the adjacent cavities 5 of the two adjacent cavity rows 7 of the cavity group 9.
As illustrated in FIG. 11, each cavity 4 (5) has an oblong shape consisting of a rectangular section 10 (11) and a pair of semicircular sections 12 (13) formed at opposite ends of the rectangular section 10 (11). The respective center axes 14 and 15 of the cavities 4 and 5 of the cavity groups 8 and 9 are inclined at the same angle .theta. to the direction of extrusion Z, hence to the axial direction Y--Y, to the left and to the right, respectively.
In mixing operation, the rotor 3 and the extruding member are driven for rotation by the motor to press a plurality of highly viscous materials, such as molten plastics or fluidized rubbers, through the inlet into the stator 2. Then, the material are successively moved in the direction Z of extrusion from the cavities 4 and 5 to the overlapping cavities 4 and 5 by the thrusting force of the extruding member. Since the rotor 3 is rotating, the materials differing from each other in quality or color are dispersed and mixed uniformly as they are transferred from the cavities 4 and 5 to the adjacent cavities 4 and 5 by the complex combined action of deflective thrust force and circumferential shearing force. However, the oblong cavities 4 and 5 of this known cavity transfer mixing extruder, extending at an angle to the axial direction have not been satisfactory with respect to the ability to advance the materials. Referring to FIG. 13 showing a portion of the development of the inner circumference of the stator 2 and that of the outer circumference of the rotor 2, the cavities 4 of the stator 2 move in the direction of the arrow R1 relative to the outer circumference of the rotor 3, while the cavities 5 of the rotor 3 move in the direction of the arrow R2 relative to the inner circumference of the stator 2. Then, the materials filling the cavities 4 and 5 are compelled to move toward the front of the cavities 4 and 5, namely, in the direction Z of extrusion, by the edges of the cavities 5 and 4. However, the materials are forced in the opposite direction in the front semicircular portions of the cavities 4 and 5, and hence the materials tend to stagnate in portion A of the cavities 4 and in portions B in the cavities 5. This phenomenon will be understood more clearly when the oblong cavities 4 and 5 are simulated by rectangular cavities 4' and 5' as illustrated in FIG. 14. The edges C corresponding to the semicircular edges of the oblong cavities 4 and 5 force the materials in a direction opposite to the direction Z of extrusion, and hence the materials tend to stagnate behind the edges C.
FIG. 32 illustrates another known cavity transfer mixing extruder having a vent for discharging moisture and volatile substances contained in the materials. This cavity transfer mixing extruder comprises a barrel 90, a screw 91 coaxially and rotatably extended within the barrel 90, a housing 94 and a stator 95 fitted in the housing 94. The barrel 90 has an inlet opening 92 at the rear end and a vent 93 at the middle. The housing 94 is joined to the front end of the barrel 90. The screw 91 has a first feed section 96 corresponding to the inlet opening 92, a first metering section 97 extending behind the vent 93, a vent section 98 corresponding to the vent 93, a second metering section 99 before the stator 95, and a rotor section 100 corresponding to the stator 95. The housing 94, the stator 95 and the rotor section 100 constitute a cavity transfer mixing unit.
Generally, the productivity of the cavity transfer mixing extruder of this type is dependent on the extruding capacity of the second extruding zone, namely, a section after the vent 93, which is reduced due to a pressure drop in the cavity transfer mixing unit joined to the front end of the extruding unit. Furthermore, since heat is generated in the cavity transfer mixing unit due to the cavity transfer mixing action of the cavity transfer mixing unit, and hence the rotating speed of the screw 91 is limited to a certain level, it has been impossible to extrude a material having a viscosity exceeding a certain level.
Still further, since the first zone extending before the vent 93 needs to be sufficiently long to heat the material so that the material is sufficiently plasticized, an elongate screw having a very large L/D ratio, for example, an L/D ratio in the range of 18 to 20 for rubber and in the range of 28 to 35 for plastics, is necessary, which is disadvantageous with respect to mechanical stability.